Why is weight painting appearing on areas of my mesh I haven't painted?
The problem is due to the fact the frame lock is not a separated part of the mesh. The vertices involved in your issue link the frame lock to the other part of your mesh. So this is a model problem not a weight paint problem.

You need to rip these vertices.

Doing that is a bit messy with this model as many tris are involved. But here is the principles :


Go to edit mode
In the modifier panel click on 'Display modifier in edit mode' and on 'adjust edit cage to modifier result' (in order to identify the good vertices easier)




So you can see that in edit mode when you move the frame lock bone :


Now the principle is to select the edge which links the two parts and rip it with V.
Then remove the unwanted vertices from the 'frame lock' group.

I had to repeat these two steps several times as the vertices are very intricated here. And that makes the detailed explanations hard to give here.

But the result :

and the blend file